The White House said on Thursday that U.S. officials were treating a cyber attack on Sony Pictures as a serious national security matter and that President Barack Obama’s National Security Council was considering a proportionate response.

Obama’s top national security officials have met daily about the attack, which was done by a “sophisticated actor,” White House spokesman Josh Earnest told reporters.

But Earnest said he was not in a position to confirm North Korea was responsible for the attack, and said the federal investigation was progressing.